As mobile devices have been increasingly developed, and the demand of such mobile devices has increased, the demand of secondary batteries has been also sharply increased as an energy source for the mobile devices. Based on their external and internal structures, the secondary batteries are generally classified into a cylindrical battery, a prismatic battery, and a pouch-shaped battery. As the mobile devices have been miniaturized, the prismatic battery and the pouch-shaped battery, which have a small width to length ratio, have attracted considerable attention recently.
The prismatic secondary battery is manufactured by placing an electrode assembly including cathodes, anodes, and separators in a prismatic battery case, mounting a base plate to the upper end of the battery case, for example, by welding, injecting electrolyte into the battery case through an electrolyte injection hole formed in the base plate, sealing the electrolyte injection hole with a metal ball, mounting a safety element and a protection circuit to the base plate, and sealing the battery case with a housing (an external case).
FIG. 1 is a plan view illustrating a base plate mounted to the upper end of a battery case of a conventional prismatic secondary battery with a vertical sectional view taken along line A-A.
Referring to FIG. 1, the base plate 100 is provided at the middle thereof with an electrode terminal 101, which is connected to an electrode tap (for example, an anode tap) of an electrode assembly. The base plate 100 is provided at one side thereof with an electrolyte injection hole 102 for allowing electrolyte to be injected therethrough. Between the electrode terminal 101 and the base plate 100 is disposed an insulating member 103 for electrically isolating the electrode terminal 101 from the base plate 200, which is connected to the other electrode tap (for example, a cathode tap) of the electrode assembly so as to serve as an electrode terminal.
The electrolyte injection hole 102, through which the electrolyte is injected, is formed generally into a vertical sectional shape of a rectangle, as shown in FIG. 2. The electrolyte injection hole 102 is sealed with a metal ball 104, for example, an aluminum ball, having a diameter slightly greater than that of the electrolyte injection hole 102. Specifically, the metal ball 104 is located on the electrolyte injection hole 102, and then the metal ball 104 is pressed from above such that the metal ball 104 is plastically deformed to seal the electrolyte injection hole 102. Subsequently, a thin metal plate is securely fixed to the base plate 100 by laser welding, whereby the electrolyte injection hole 102 is completely sealed.
When the metal ball is plastically deformed and is inserted into the electrolyte injection hole, which is formed in the vertical sectional shape of a rectangle, however, the lower circumferential part of the metal ball is pushed outward by the inside upper end of the electrolyte injection hole. As a result, a groove is formed on the metal ball. The formed groove badly affects the sealing of the electrolyte injection hole by laser welding. Furthermore, inside cracks are continuously generated along the interface between the metal ball and the inside surface of the electrolyte injection hole due to formation of the groove, and therefore, the sealability of the electrolyte injection hole is lowered.
In order to solve the above-mentioned problems, Japanese Unexamined Patent Publication No. 2004-0023253 and Japanese Unexamined Patent Publication No. 2002-358948 disclose a method of forming the inside upper end of the electrolyte injection hole in a taper structure and inserting a sealing member having a shape corresponding to the electrolyte injection hole, instead of the metal ball, into the electrolyte injection hole so as to seal the electrolyte injection hole. This sealing structure has an effect of preventing the generation of the above-mentioned groove. However, as compared with the case that the sealing member (for example, the metal ball) is plastically deformed to seal the electrolyte injection hole, this sealing structure has problems in that the coupling force between the electrolyte injection hole and the sealing member is small, and therefore, the sealability of the electrolyte injection hole is low. For this reason, additional operations must be further carried out so as to complement the small coupling force, which is troublesome.
Consequently, the necessity of a technology to fundamentally solve the above-mentioned problems is highly increased.